Gut microbiota has long been thought to contribute to inflammatory diseases, and multiple reports in animal models and humans suggest that antibiotic treatment alters autoimmune disease manifestations. We have recently demonstrated in rodents that specific microbes induce the differentiation of Th17 cells in the intestinal lamina propria. There is strong genetic and therapy-based evidence that "pro-inflammatory" Th17 and "anti- inflammatory" regulatory T cells (Treg) have critical roles in autoimmune diseases, including rheumatoid arthritis (RA), psoriatic arthritis (PsA), and Crohn's disease. We propose to study the role of gut (intestinal and oral) microbiota in RA and other inflammatory arthritides. Our primary hypotheses are that: 1) characterization of Th17-inducing microbes in human intestine will provide insight into disease pathogenesis;and 2) directed manipulation of the gut microbiota will result in alteration of arthritis biomarkers, including Th17/Treg balance. Insights attained may elucidate how the T cell network responds to microbial interactions with host intestinal components and provide a rationale for the development of new therapeutic approaches for RA. Three Specific Aims are proposed: 1) To create a multidisciplinary center to characterize human gut microbiome in patients with RA and related conditions. 2) To employ Th17-dependent mouse models of RA to study the role of microbiota/T cell interactions in development of disease, to directly assess whether specific bacteria in RA patients can be implicated in disease pathogenesis. Both direct bacterial cocktails and bacteria identified in RA patients will be inoculated into the mice. 3) To study the role of human gut microbiota in RA pathogenesis by: a) cross-sectional study to determine whether a specific taxon or bacterial family in the human gut is associated with RA or PsA;b) clinical and blood examinations to assess baseline disease activity, genetic predisposition and immune cellular function of arthritis patients vs controls;c) prospective, interventional proof of concept biomarker study to determine whether alteration of the gut microbiota normalizes cellular immune functions in patients with RA. We will compare 2 antibiotic regimens to assess whether therapy induces i) characteristic changes in the gut microbiome (including changes in abundance of specific target taxons), and ii) alterations in immune biomarkers, particularly Th17 and Treg cell levels and/or function. Carefully selected outcomes should permit us to correlate the presence of a specific microorganism or microbiome pattern with changes in cellular immune response, other specific biomarkers, and clinical activity. Relevance: This project is consistent with the goals of the Human Microbiome Project, a major NIH Roadmap initiative, and has the potential to be truly transformative by filling a fundamental knowledge gap regarding the cause of inflammatory arthritis. The results could transform our understanding of the relationships between microbes and humans, and lead to innovative diagnostic tests and future treatments.